Internet Engineering Task Force (IETF) C. Daboo

Internet Engineering Task Force (IETF) C. Daboo
Request for Comments: 7529 Apple Inc.
Updates: 5545, 6321, 7265 G. Yakushev
Category: Standards Track Google Inc.
ISSN: 2070-1721 May 2015

Non-Gregorian Recurrence Rules in the Internet Calendaring and
Scheduling Core Object Specification (iCalendar)

Abstract

This document defines extensions to the Internet Calendaring and
Scheduling Core Object Specification (iCalendar) (RFC 5545) to
support use of non-Gregorian recurrence rules. It also defines how
Calendaring Extensions to WebDAV (CalDAV) (RFC 4791) servers and
clients can be extended to support these new recurrence rules.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7529.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Daboo & Yakushev Standards Track [Page 1]

RFC 7529 iCalendar RSCALE Extension May 2015

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Extended RRULE Property . . . . . . . . . . . . . . . . . . . 6
4.1. Skipping Invalid Dates . . . . . . . . . . . . . . . . . 6
4.2. Handling Leap Months . . . . . . . . . . . . . . . . . . 9
4.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Registering Calendar Systems . . . . . . . . . . . . . . . . 13
6. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Use with iTIP . . . . . . . . . . . . . . . . . . . . . . . . 14
8. Use with xCal . . . . . . . . . . . . . . . . . . . . . . . . 15
9. Use with jCal . . . . . . . . . . . . . . . . . . . . . . . . 15
10. Use with CalDAV . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. CALDAV:supported-rscale-set Property . . . . . . . . . . 17
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
12.1. Normative References . . . . . . . . . . . . . . . . . . 18
12.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. xCal RELAX NG Schema Update . . . . . . . . . . . . 20
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21

1. Introduction

The iCalendar [RFC5545] data format is in widespread use to represent
calendar data. iCalendar represents dates and times using the
Gregorian calendar system only. It does provide a way to use non-
Gregorian calendar systems via a "CALSCALE" property, but this has
never been used. However, there is a need to support at least non-
Gregorian recurrence patterns to cover anniversaries, and many local,
religious, or civil holidays based on non-Gregorian dates.

There are several disadvantages to using the existing "CALSCALE"
property in iCalendar for implementing non-Gregorian calendars:

1. The "CALSCALE" property exists in the top-level "VCALENDAR"
objects and thus applies to all components within that object.
In today's multi-cultural society, that restricts the ability to
mix events from different calendar systems within the same
iCalendar object, e.g., it would prevent having both the
Gregorian New Year and Chinese New Year in the same iCalendar
object.

2. Time zone and daylight saving time rules are typically published
using Gregorian calendar dates and rules (e.g., "the 3rd Sunday
in March") and are thus converted to iCalendar "VTIMEZONE"

Daboo & Yakushev Standards Track [Page 2]

RFC 7529 iCalendar RSCALE Extension May 2015

components using Gregorian dates and recurrence rules. This
results in the problem whereby one component (the "VTIMEZONE") is
fixed to the Gregorian calendar system, and another (a "VEVENT")
wants to use a different non-Gregorian calendar scale; thus, the
single top-level "CALSCALE" property is again inadequate.

This specification solves these issues by allowing the "CALSCALE" to
remain set to Gregorian but redefining the "RRULE" recurrence rule
property to accept new items, including one that allows non-Gregorian
calendar systems to be used. With this, all the "DATE", "DATE-TIME",
and "PERIOD" values in the iCalendar object would remain specified
using the Gregorian calendar system, but repeating patterns in other
calendar systems could be defined. It is then up to calendar user
agents and servers to map between Gregorian and non-Gregorian
calendar systems in order to expand out recurrence instances. The
non-Gregorian recurrence rules can be used in any iCalendar component
that allows the "RRULE" property to be specified, including
"VTIMEZONE" components (to allow for possible future use of non-
Gregorian rules in published daylight saving time data).

This specification does not itself define calendar systems; rather,
it utilizes the calendar system registry defined by the Unicode
Consortium in their Common Locale Data Repository (CLDR) project
[UNICODE.CLDR], as implemented in the Unicode (International
Components for Unicode (ICU)) Library [UNICODE.ICU].

This specification makes the following updates:

It updates iCalendar [RFC5545], the XML format for iCalendar
(xCal) [RFC6321], and the JSON format for iCalendar (jCal)
[RFC7265], to extend the "RRULE" property definition.

It clarifies use of the iCalendar Transport-Independent
Interoperability Protocol (iTIP) [RFC5546] to specify how the
extended "RRULE" property should be handled in iTIP messages.

It extends CalDAV [RFC4791] to specify how the extended "RRULE"
property can be supported by CalDAV servers and clients.

2. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].

Daboo & Yakushev Standards Track [Page 3]

RFC 7529 iCalendar RSCALE Extension May 2015

The notation used in this memo is the ABNF notation of [RFC5234] as
used by iCalendar [RFC5545]. Any syntax elements shown below that
are not explicitly defined in this specification come from iCalendar
[RFC5545], iTIP [RFC5546], and CalDAV [RFC4791].

When XML element types in the namespaces "DAV:" and
"urn:ietf:params:xml:ns:caldav" are referenced in this document
outside of the context of an XML fragment, the string "DAV:" and
"CALDAV:" will be prefixed to the element type names, respectively.

When a Gregorian calendar date is shown in text, it will use the
format "YYYYMMDD", where "YYYY" is the 4-digit year, "MM" the 2-digit
month, and "DD" the 2-digit day (this is the same format used in
iCalendar [RFC5545]). The Chinese calendar will be used as an
example of a non-Gregorian calendar for illustrative purposes. When
a Chinese calendar date is shown in text, it will use the format
"{C}YYYYMM[L]DD" -- i.e., the same format as Gregorian but with a
"{C}" prefix, and an optional "L" character after the month element
to indicate a leap month. Similarly, {E} and {H} are used in other
examples as prefixes for Ethiopic (Amete Mihret) and Hebrew dates,
respectively. The "{}" prefix is used for purely illustrative
purposes and never appears in actual dates used in iCalendar or
related specifications. Note that the Chinese calendar years shown
in the examples are based on the Unicode (ICU) [UNICODE.ICU]
library's Chinese calendar epoch. While there are several different
Chinese calendar epochs in common use, the choice of one over another
does not impact the actual calculation of the Gregorian equivalent
dates, provided conversion is always done using the same epoch.

3. Overview

In the Gregorian calendar system, each year is composed of a fixed
number of months (12), with each month having a fixed number of days
(between 30 and 31), except for the second month (February), which
contains either 28 or 29 days (the latter in a leap year). Weeks are
composed of 7 days, with day names Monday, Tuesday, Wednesday,
Thursday, Friday, Saturday, and Sunday. Years can have either 365 or
366 days (the latter in a leap year). The number of whole weeks in a
year is 52 (though the [ISO.8601.2004] week numbering scheme used by
iCalendar [RFC5545] can have a numeric count up to 53).

In iCalendar, the "RECUR" value type defines various fields used to
express a recurrence pattern, and those fields are given limits based
on those of the Gregorian calendar system. Since other calendar
systems can have different limits and other behaviors that need to be
accounted for, the maximum values for the elements in the "RECUR"
value are not covered by this specification.

Daboo & Yakushev Standards Track [Page 4]

RFC 7529 iCalendar RSCALE Extension May 2015

To generate a set of recurring instances in a non-Gregorian calendar
system, the following principles are used:

1. iCalendar data continues to use the "GREGORIAN" calendar system,
so all "DATE", "DATE-TIME", and "PERIOD" values continue to use
the Gregorian format and limits.

2. The "RRULE" property is extended to include an "RSCALE" element
in its value that specifies the calendar system to use for the
recurrence pattern. The existing elements of the "RRULE" value
type are used, but modified to support different upper limits,
based on the "RSCALE" value, as well as a modification to month
numbers to allow a leap month to be specified. Existing
requirements for the use of "RRULE" all still apply (e.g., the
"RRULE" has to match the "DTSTART" value of the master instance).
Other recurrence properties such as "RECURRENCE-ID", "RDATE", and
"EXDATE" continue to use the Gregorian date format as "CALSCALE"
is unchanged.

When generating instances, the following procedure might be used:

1. Convert the "DTSTART" property value of the master recurring
component into the date and time components for the calendar
system specified by the "RSCALE" element in the "RRULE" value.
This provides the "seed" value for generating subsequent
recurrence instances.

2. Iteratively generate instances using the "RRULE" value applied to
the year, month, and day components of the date in the new
calendar system.

3. For each generated instance, convert the dates and times back
from the non-Gregorian form into Gregorian and use those values
for other properties such as "RECURRENCE-ID".

Consider the following example for an event representing the Chinese
New Year:

DTSTART;VALUE=DATE:20130210
RRULE:RSCALE=CHINESE;FREQ=YEARLY
SUMMARY:Chinese New Year

To generate instances, first the "DTSTART" value "20130210" is
converted into the Chinese calendar system giving "{C}46500101".
Next, the year component is incremented by one to give "{C}46510101",
and that is then converted back into Gregorian as "20140131".
Additional instances are generated by iteratively increasing the year
component in the Chinese date and converting back to Gregorian.

Daboo & Yakushev Standards Track [Page 5]

RFC 7529 iCalendar RSCALE Extension May 2015

4. Extended RRULE Property

This specification extends the existing "RRULE" iCalendar property
value to include a new "RSCALE" element that can be used to indicate
the calendar system used for generating the recurrence pattern.

When "RSCALE" is present, the other changes to "RRULE" are:

1. Elements that include numeric values (e.g., "BYYEARDAY") have
numeric ranges defined by the "RSCALE" value (i.e., in some
calendar systems there might be more than 366 days in a year).

2. Month numbers can include an "L" suffix to indicate that the
specified month is a leap month in the corresponding calendar
system (see Section 4.2).

3. A "SKIP" element is added to define how "missing" instances are
handled (see Section 4.1).

The syntax for the "RECUR" value is modified in the following
fashion:

; recur-rule-part is extended from RFC 5545
recur-rule-part =/ ("RSCALE" "=" rscale)
/ ("SKIP" "=" skip)

rscale = (iana-token ; A CLDR-registered calendar system
; name.
/ x-name) ; A non-standard, experimental
; calendar system name.
; Names are case insensitive,
; but uppercase values are preferred.

skip = ("OMIT" / "BACKWARD" / "FORWARD")
; Optional, with default value "OMIT", and
; MUST NOT be present unless "RSCALE" is present.

monthnum = 1*2DIGIT ["L"]
; Existing element modified to include a leap
; month indicator suffix.

4.1. Skipping Invalid Dates

In every calendar system, only certain combinations of day-of-month
and month values are valid for a given year, e.g., in the Gregorian
calendar system, January 31st is valid but February 31st is not.
Similarly, February 29th is valid in a leap year but invalid in a
non-leap year. Other calendar systems can also include leap months

Daboo & Yakushev Standards Track [Page 6]

RFC 7529 iCalendar RSCALE Extension May 2015

(see Section 4.2), which vary from year to year. This poses a
problem for recurring events where the frequency of recurrence might
give rise to an invalid date. For example, a recurring event that
starts on January 31st and is set to repeat monthly will generate
invalid dates for months with fewer than 31 days. The iCalendar
[RFC5545] specification requires recurrence rule generators to ignore
any invalid dates generated when iterating the rule. However, that
behavior might be surprising to a calendar user born on a leap day
and whose birthday event only appears on their calendar every four
years. There are common conventions used by humans to determine what
to do in such cases, but those conventions can differ from calendar
system to calendar system, as well as within the same calendar
system, depending on the nature of the event. Typically, humans will
expect the "missing" events to be moved to an earlier or later
(valid) date.

This specification introduces a new "RRULE" element, "SKIP", for use
only when the "RSCALE" element is present. The "SKIP" element allows
the calendar user agent to specify new options for handling invalid
dates.

"SKIP=OMIT": this is the default option and corresponds to the
existing iCalendar behavior of simply ignoring the invalid date.

"SKIP=BACKWARD": when this option is set, a date with an invalid
month is changed to the previous (valid) month. A date with an
invalid day-of-month is changed to the previous (valid)
day-of-month.

"SKIP=FORWARD": when this option is set, a date with an invalid
month is changed to the next (valid) month. A date with an
invalid day-of-month is changed to the next (valid) day-of-month.

Note that for both "BACKWARD" and "FORWARD", if the month is changed
and results in an invalid day-of-month, then the skip behavior will
be reapplied as per the day-of-month rules, according to the
processing order defined below.

The month and day-of-month skip behavior is only applied at specific
points during the processing of an "RRULE" as determined by the order
in which any "BYxxx" elements are processed. The order is as follows
(based on the "RRULE" element processing order defined in
Section 3.3.10 of [RFC5545]):

o BYMONTH

o SKIP (for invalid month only)

Daboo & Yakushev Standards Track [Page 7]

RFC 7529 iCalendar RSCALE Extension May 2015

o BYWEEKNO

o BYYEARDAY

o BYMONTHDAY

o SKIP (for invalid day)

o BYDAY

o BYHOUR

o BYMINUTE

o BYSECOND

o BYSETPOS

o COUNT

o UNTIL

It is often possible to avoid having to deal with invalid dates by
determining the real intent of a human user, e.g., a human creating a
monthly recurring event that starts on January 31st likely intends
the event to occur on the last day of every month, in which case that
could be encoded into an "RRULE" by using the "BYMONTHDAY=-1"
element.

Only a few types of recurrence patterns are likely to need the use of
"SKIP". The following is a list of some situations where it might be
needed:

1. The start date of the recurrence is a leap day in the specified
calendar system.

2. The start date of the recurrence is in a leap month in the
specified calendar system.

3. The start date of the recurrence has a day-of-month value greater
than the smallest day-of-month value for any month in any year in
the specified calendar system.

4. A "BYMONTHDAY" element in an "RRULE" has a day-of-month value
greater than the smallest day-of-month value for any month in any
year in the specified calendar system.

Daboo & Yakushev Standards Track [Page 8]

RFC 7529 iCalendar RSCALE Extension May 2015

5. A "BYMONTH" element in an "RRULE" has a value corresponding to a
leap month in the specified calendar system.

6. A combination of a "BYMONTHDAY" element and a "BYMONTH" element
in an "RRULE" has a value corresponding to a leap day in the
specified calendar system.

7. A "BYYEARDAY" element in an "RRULE" has an absolute value greater
than the smallest number of days in any year in the specified
calendar system.

8. A "BYWEEKNO" element in an "RRULE" has an absolute value greater
than the smallest number of weeks in any year in the specified
calendar system.

Examples of using "SKIP" for some common use cases appear in
Section 4.3.

4.2. Handling Leap Months

Leap months can occur in different calendar systems. For such
calendar systems, the following rules are applied for "identifying"
months:

1. Numeric values 1 through N are used to identify regular, non-
leap, months (where N is the number of months in a regular, non-
leap, year).

2. The suffix "L" is added to the regular month number to indicate a
leap month that follows the regular month, e.g., "5L" is a leap
month that follows the 5th regular month in the year.

Care has to be taken when mapping the month identifiers used here
with those of any underlying calendar system library being used. In
particular, the Hebrew calendar system used by Unicode (ICU)
[UNICODE.ICU] uses a month number scheme of 1 through 13, with month
6 being the leap month, and in non-leap years, month 6 is skipped.
Thus, ICU months 1 through 5 map to iCalendar months 1 through 5, ICU
month 6 maps to iCalendar month "5L", and ICU months 7 through 13 map
to iCalendar months 6 through 12.

Daboo & Yakushev Standards Track [Page 9]

RFC 7529 iCalendar RSCALE Extension May 2015

4.3. Examples

4.3.1. Chinese New Year

Consider the following set of iCalendar properties (from the example
above):

DTSTART;VALUE=DATE:20130210
RRULE:RSCALE=CHINESE;FREQ=YEARLY
SUMMARY:Chinese New Year

These define a recurring event for the Chinese New Year, with the
first instance being the one in Gregorian year 2013.

The Chinese date corresponding to the first instance is
"{C}46500101". The table below shows the initial instance and the
next four, each of which is determined by adding the appropriate
amount to the year component of the Chinese date. Also shown is the
conversion back to the Gregorian date:

+--------------+--------------------------+
| Chinese Date | Gregorian Date |
+--------------+--------------------------+
| {C}46500101 | 20130210 - DTSTART value |
| {C}46510101 | 20140131 |
| {C}46520101 | 20150219 |
| {C}46530101 | 20160208 |
| {C}46540101 | 20170128 |
+--------------+--------------------------+

4.3.2. Ethiopic 13th Month

Consider the following set of iCalendar properties:

DTSTART;VALUE=DATE:20130906
RRULE:RSCALE=ETHIOPIC;FREQ=MONTHLY;BYMONTH=13
SUMMARY:First day of 13th month

These define a recurring event for the first day of the 13th month,
with the first instance being the one in Gregorian year 2013. While
there are a number of alternative ways of writing the "RRULE" above
to achieve the same pattern of recurring dates, the one above was
chosen to illustrate a "BYMONTH" value exceeding the limit of 12,
previously described in iCalendar (Section 3.3.10 of [RFC5545]).

Daboo & Yakushev Standards Track [Page 10]

RFC 7529 iCalendar RSCALE Extension May 2015

The Ethiopic date corresponding to the first instance is
"{E}20051301". The table below shows the initial instance and the
next four, each of which is determined by adding the appropriate
amount to the year component of the Ethiopic date. Also shown is the
conversion back to the Gregorian date:

+---------------+--------------------------+
| Ethiopic Date | Gregorian Date |
+---------------+--------------------------+
| {E}20051301 | 20130906 - DTSTART value |
| {E}20061301 | 20140906 |
| {E}20071301 | 20150906 |
| {E}20081301 | 20160906 |
| {E}20091301 | 20170906 |
+---------------+--------------------------+

Note that in this example, the value of the "BYMONTH" component in
the "RRULE" matches the Ethiopic month value and not the Gregorian
month.

4.3.3. Hebrew Anniversary Starting in a Leap Month

Consider the following set of iCalendar properties:

DTSTART;VALUE=DATE:20140208
RRULE:RSCALE=HEBREW;FREQ=YEARLY;BYMONTH=5L;BYMONTHDAY=8;SKIP=FORWARD
SUMMARY:Anniversary

These define a recurring event for the 8th day of the Hebrew month of
Adar I (the leap month identified by "5L"), with the first instance
being the one in Gregorian year 2014.

The Hebrew date corresponding to the first instance is
"{H}577405L08", which is a leap month in year 5774. The table below
shows the initial instance and the next four, each of which is
determined by adding the appropriate amount to the year component of
the Hebrew date, taking into account that only year 5776 is a leap
year. Thus, in other years the Hebrew month component is adjusted
forward to month 6. Also shown is the conversion back to the
Gregorian date:

Daboo & Yakushev Standards Track [Page 11]

RFC 7529 iCalendar RSCALE Extension May 2015

+--------------+--------------------------+
| Hebrew Date | Gregorian Date |
+--------------+--------------------------+
| {H}577405L08 | 20140208 - DTSTART value |
| {H}57750608 | 20150227 |
| {H}577605L08 | 20160217 |
| {H}57770608 | 20170306 |
| {H}57780608 | 20180223 |
+--------------+--------------------------+

4.3.4. Gregorian Leap Day with SKIP

Consider the following set of iCalendar properties:

DTSTART;VALUE=DATE:20120229
RRULE:FREQ=YEARLY
SUMMARY:Anniversary

These define a recurring event for the 29th of February, 2012, in the
standard iCalendar calendar scale -- Gregorian. The standard
iCalendar behavior is that non-existent dates in a recurrence set are
ignored. Thus, the properties above would only generate instances in
leap years (2016, 2020, etc.), which is likely not what users expect.
The new "RSCALE" option defined by this specification provides the
"SKIP" element, which can be used to "fill in" the missing instances
in an appropriate fashion. The set of iCalendar properties below
does that:

DTSTART;VALUE=DATE:20120229
RRULE:RSCALE=GREGORIAN;FREQ=YEARLY;SKIP=FORWARD
SUMMARY:Anniversary

With these properties, the "missing" instances in non-leap years now
appear on the 1st of March in those years:

+-------------------------------+----------------------------+
| Instances (with SKIP=FORWARD) | Instances (without RSCALE) |
+-------------------------------+----------------------------+
| 20120229 | 20120229 - DTSTART value |
| 20130301 | |
| 20140301 | |
| 20150301 | |
| 20160229 | 20160229 |
| 20170301 | |
+-------------------------------+----------------------------+

Daboo & Yakushev Standards Track [Page 12]

RFC 7529 iCalendar RSCALE Extension May 2015

5. Registering Calendar Systems

This specification uses the Unicode Consortium's registry of calendar
systems [UNICODE.CLDR] to define valid values for the "RSCALE"
element of an "RRULE". Note that the underscore character "_" is
never used in CLDR-based calendar system names. New values can be
added to this registry following Unicode Consortium rules. It is
expected that many implementations of non-Gregorian calendars will
use software libraries provided by Unicode (ICU) [UNICODE.ICU], and
hence it makes sense to reuse their registry rather than creating a
new one. "RSCALE" values are case insensitive, but uppercase is
preferred.

CLDR supports the use of "alias" values as alternative names for
specific calendar systems. These alias values can be used as
"RSCALE" values and are treated the same as the equivalent CLDR
calendar system they are an alias for.

When using the CLDR data, calendar agents SHOULD take into account
the "deprecated" value and use the alternative "preferred" calendar
system. In particular, the "islamicc" calendar system is considered
deprecated in favor of the "islamic-civil" calendar system.

6. Compatibility

For calendar user agents that do not support the "RSCALE" element, or
do not support the calendar system specified by the "RSCALE" element
value, the following behaviors are possible when processing iCalendar
data:

1. The calendar user agent can reject the entire iCalendar object
within which at least one iCalendar component uses the
unrecognized "RSCALE" element or element value.

2. The calendar user agent can reject just the iCalendar components
containing an unrecognized "RSCALE" element or element value.
Note that any overridden components associated with those
rejected components MUST also be rejected (i.e., any other
components with the same "UID" property value as the one with the
unrecognized "RSCALE" element or element value).

3. The calendar user agent can fall back to a non-recurring behavior
for the iCalendar component containing the unrecognized "RSCALE"
element or element value (effectively ignoring the "RRULE"
property). However, any overridden components SHOULD be rejected
as they would represent "orphaned" instances that would seem to
be out of place.

Daboo & Yakushev Standards Track [Page 13]

RFC 7529 iCalendar RSCALE Extension May 2015

In general, the best choice for a calendar user agent would be option
(2) above, as it would be the least disruptive choice. Note that
when processing iTIP [RFC5546] messages, the manner of the rejection
is covered as discussed in the next section.

7. Use with iTIP

iTIP [RFC5546] defines how iCalendar data can be sent between
calendar user agents to schedule calendar components between calendar
users. It is often not possible to know the capabilities of a
calendar user agent to which an iTIP message is being sent, but iTIP
defines fallback behavior in such cases.

For calendar user agents that do not support the "RSCALE" element,
the following can occur when iTIP messages containing an "RSCALE"
element are received:

The receiving calendar user agent can reject the entire iTIP
message and return an iTIP reply with a "REQUEST-STATUS" property
set to the "3.1" status code (as per Section 3.6.14 of [RFC5546]).

The receiving calendar user agent can fall back to a non-recurring
behavior for the calendar component (effectively ignoring the
"RRULE" property) and return an iTIP reply with a "REQUEST-STATUS"
property set to the "2.3", "2.5", "2.8", or "2.10" status codes
(as per Sections 3.6.4, 3.6.6, 3.6.9, or 3.6.11, respectively, of
[RFC5546]).

For calendar user agents that support the "RSCALE" element but do not
support the calendar system specified by the "RSCALE" element value,
the following can occur:

The iTIP message SHOULD be rejected, returning a "REQUEST-STATUS"
property set to the "3.1" status code (as per Section 3.6.14 of
[RFC5546]).

If the iTIP message is accepted and the calendar component treated
as non-recurring, an iTIP reply with a "REQUEST-STATUS" property
set to the "2.8" or "2.10" status codes (as per Sections 3.6.9 or
3.6.11, respectively, of [RFC5546]) SHOULD be returned.

As noted in Section 6, the best choice is to reject the entire iTIP
message.

Daboo & Yakushev Standards Track [Page 14]

RFC 7529 iCalendar RSCALE Extension May 2015

8. Use with xCal

xCal [RFC6321] defines how iCalendar data is represented in XML.
This specification extends the <recur> XML element in Section 3.6.10
of [RFC6321] in the following manner:

1. A new <rscale> XML element is defined as a child element of
<recur>. The content of this element MUST be a string whose
value is the "RSCALE" element value of the "RRULE", with case
preserved.

2. A new <skip> XML element is defined as a child element of
<recur>. The content of this element MUST be a string whose
value is the "SKIP" element value of the "RRULE", with case
preserved.

3. The <bymonth> XML element is redefined to support either numeric
or string values as its content (as per Section 4.2).

Extensions to the RELAX NG schema in Appendix A of [RFC6321] are
defined in Appendix A of this document.

Example: the iCalendar "RRULE" property:

RRULE:RSCALE=GREGORIAN;FREQ=YEARLY;SKIP=FORWARD

would be represented in XML as:

<rrule>
<recur>
<rscale>GREGORIAN</rscale>
<freq>YEARLY</freq>
<skip>FORWARD</skip>
</recur>
</rrule>

9. Use with jCal

jCal [RFC7265] defines how iCalendar data is represented in JSON.
This specification extends the "recur" JSON object defined in
Section 3.6.10 of [RFC7265] in the following manner:

1. A new "rscale" child member is defined. This MUST be a string
whose value is the "RSCALE" element value of the "RRULE", with
case preserved.

Daboo & Yakushev Standards Track [Page 15]

RFC 7529 iCalendar RSCALE Extension May 2015

2. A new "skip" child member is defined. This MUST be a string
whose value is the "SKIP" element value of the "RRULE", with case
preserved.

3. The "bymonth" child member is redefined to support either numeric
or string values. If the "BYMONTH" element value is an integer,
then a numeric JSON value MUST be used. If the "BYMONTH" element
value is an integer with the "L" suffix (as per Section 4.2),
then a JSON string value MUST be used.

Example: the iCalendar "RRULE" property:

RRULE:RSCALE=GREGORIAN;FREQ=YEARLY;SKIP=FORWARD

would be represented in JSON as:

[
"rrule",
{},
"recur",
{
"rscale": "GREGORIAN",
"freq": "YEARLY",
"skip": "FORWARD"
}
]

10. Use with CalDAV

The CalDAV [RFC4791] calendar access protocol allows clients and
servers to exchange iCalendar data. In addition, CalDAV clients are
able to query calendar data stored on the server, including time-
based queries. Since an "RSCALE" element value determines the time
ranges for recurring instances in a calendar component, CalDAV
servers need to support it to interoperate with clients also using
the "RSCALE" element.

A CalDAV server advertises a CALDAV:supported-rscale-set Web
Distributed Authoring and Versioning (WebDAV) property on calendar
home or calendar collections if it supports use of the "RSCALE"
element as described in this specification. The server can advertise
a specific set of supported calendar systems by including one or more
CALDAV:supported-rscale XML elements within the
CALDAV:supported-rscale-set XML element. If no
CALDAV:supported-rscale XML elements are included in the WebDAV
property, then clients can try any calendar system value but need to
be prepared for a failure when attempting to store the calendar data.

Daboo & Yakushev Standards Track [Page 16]

RFC 7529 iCalendar RSCALE Extension May 2015

Clients MUST NOT attempt to store iCalendar data containing "RSCALE"
elements if the CALDAV:supported-rscale-set WebDAV property is not
advertised by the server.

The server SHOULD return an HTTP 403 response with a DAV:error
element containing a CALDAV:supported-rscale XML element, if a client
attempts to store iCalendar data with an "RSCALE" element value not
supported by the server.

It is possible for an "RSCALE" value to be present in calendar data
on the server being accessed by a client that does not support an
"RSCALE" element or its specified value. It is expected that
existing clients, unaware of "RSCALE", will fail gracefully by
ignoring the calendar component, while still processing other
calendar data on the server (as per option (2) in Section 6).

10.1. CALDAV:supported-rscale-set Property

Name: supported-rscale-set

Namespace: urn:ietf:params:xml:ns:caldav

Purpose: Enumerates the set of supported iCalendar "RSCALE" element
values supported by the server.

Protected: This property MUST be protected and SHOULD NOT be
returned by a PROPFIND allprop request (as defined in Section 14.2
of [RFC4918]).

Description: See above.

Definition:

<!ELEMENT supported-rscale-set (supported-rscale*)>
<!ELEMENT supported-rscale (#PCDATA)>


Example:

<C:supported-rscale-set
xmlns:C="urn:ietf:params:xml:ns:caldav">
<C:supported-rscale>GREGORIAN</C:supported-rscale>
<C:supported-rscale>CHINESE</C:supported-rscale>
<C:supported-rscale>ISLAMIC-CIVIL</C:supported-rscale>
<C:supported-rscale>HEBREW</C:supported-rscale>
<C:supported-rscale>ETHIOPIC</C:supported-rscale>
</C:supported-rscale-set>

Daboo & Yakushev Standards Track [Page 17]

RFC 7529 iCalendar RSCALE Extension May 2015

11. Security Considerations

This specification does not introduce any additional security
concerns beyond those described in [RFC5545], [RFC5546], and
[RFC4791].

12. References

12.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault,
"Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
March 2007, <http://www.rfc-editor.org/info/rfc4791>.

[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007,
<http://www.rfc-editor.org/info/rfc4918>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.

[RFC5545] Desruisseaux, B., Ed., "Internet Calendaring and
Scheduling Core Object Specification (iCalendar)", RFC
5545, September 2009,
<http://www.rfc-editor.org/info/rfc5545>.

[RFC5546] Daboo, C., Ed., "iCalendar Transport-Independent
Interoperability Protocol (iTIP)", RFC 5546, December
2009, <http://www.rfc-editor.org/info/rfc5546>.

[RFC6321] Daboo, C., Douglass, M., and S. Lees, "xCal: The XML
Format for iCalendar", RFC 6321, August 2011,
<http://www.rfc-editor.org/info/rfc6321>.

[RFC7265] Kewisch, P., Daboo, C., and M. Douglass, "jCal: The JSON
Format for iCalendar", RFC 7265, May 2014,
<http://www.rfc-editor.org/info/rfc7265>.

[UNICODE.CLDR]
The Unicode Consortium, "CLDR calendar.xml Data", Unicode
Consortium CLDR,
<http://www.unicode.org/repos/cldr/tags/latest/common/
bcp47/calendar.xml>.

Daboo & Yakushev Standards Track [Page 18]

RFC 7529 iCalendar RSCALE Extension May 2015

12.2. Informative References

[ISO.8601.2004]
International Organization for Standardization, "Data
elements and interchange formats - Information interchange
- Representation of dates and times", ISO Standard 8601,
December 2004.

[UNICODE.ICU]
"International Components for Unicode", April 2014,
<http://site.icu-project.org>.

Daboo & Yakushev Standards Track [Page 19]

RFC 7529 iCalendar RSCALE Extension May 2015

Appendix A. xCal RELAX NG Schema Update

The following changes are made to the RELAX NG schema defined in
Appendix A of [RFC6321].

# 3.3.10 RECUR
# This extension adds type-rscale and type-skip,
# and modifies type-bymonth

value-recur = element recur {
type-rscale?,
type-freq,
(type-until | type-count)?,
element interval {
xsd:positiveInteger
}?,
type-bysecond*,
type-byminute*,
type-byhour*,
type-byday*,
type-bymonthday*,
type-byyearday*,
type-byweekno*,
type-bymonth*,
type-bysetpos*,
element wkst { type-weekday }?,
type-skip?
}

type-rscale = element rscale {
xsd:string
}

type-bymonth = element bymonth {
xsd:positiveInteger |
xsd:string
}

type-skip = element skip {
"OMIT" |
"BACKWARD" |
"FORWARD"
}

Daboo & Yakushev Standards Track [Page 20]

RFC 7529 iCalendar RSCALE Extension May 2015

Acknowledgments

Thanks to the following for feedback: Mark Davis, Mike Douglass,
Donald Eastlake, Peter Edberg, Marten Gajda, Philipp Kewisch, Barry
Leiba, Jonathan Lennox, Ken Murchison, Arnaud Quillaud, Dave Thewlis,
and Umaoka Yoshito.

This specification originated from work at the Calendaring and
Scheduling Consortium, which has helped with the development and
testing of implementations.

Authors' Addresses

Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
United States

EMail: [email protected]
URI: http://www.apple.com/

Gregory Yakushev
Google Inc.
Brandschenkestrasse 100
8002 Zurich
Switzerland

EMail: [email protected]
URI: http://www.google.com/

Daboo & Yakushev Standards Track [Page 21]